fix(ui): improve ThemeToggle icon contrast

[RUKAYAT-CODER]

Summary

Fixes the ThemeToggle icon contrast issue by replacing the hardcoded �g-[#232329] background with theme-aware Tailwind utility classes.

Changes

• src/components/ui/ThemeToggle.tsx: replaced hardcoded dark background with �g-gray-100 dark:bg-gray-800 and matching hover/text colors for proper contrast in both light and dark modes; added �ria-hidden to icons
• src/tests/components/theme-toggle-contrast.test.tsx: added invariant tests verifying no hardcoded bg, theme-aware classes present, and icons are hidden from assistive technology

closes #197